The research proposed here is directed toward an understanding of the molecular mechanism of general genetic recombination. This goal is approached through the study of the structure of special sites in DNA promoting a high rate of recombination in their vicinity, and through the identification and study of the activity of the protein which recognizes these sites. The Chi recombination hotspots of bacteriophage lambda and its host Escherichia coli will be studied in the proposed research. In addition, proteins such as RecA and RecBC known to act in the pathway stimulated by Chi, will be studied. We have determined that the nucleotide sequence 5' G-C-T-G-G-T-G-G 3' (or its complement, or both) defines Chi. We will search for a protein which recognizes this sequence via a study of cell mutants in which Chi has reduced activity. We will study proteins which bind to Chi and search for a biochemical activity stimulated by Chi. In collaborative work, we plan to synthesize Chi by chemical means. Other proposed work is aimed at determining the distribution of Chi in a segment of the E. coli chromosome and determining whether these Chi sites, in the absence of lambda, act during cellular recombination. We have studied the mechanism by which RecBc enzyme unwinds DNA to produce single-stranded loops and subsequently rewinds these loops. Our current results suggest that RecA protein can use these single-stranded loops in promoting chromosome fusions. We will study the structure of these fusions, how they are formed, and how they might procede toward recombinant chromosomes.